The present invention is a communication method and apparatus for communicating with tags in a communication region and particularly for communicating to locate, track and identify tags in a communication region. The present invention is particularly useful where large numbers of tags are present in the communication region, where the locations or identities of the tags in the communication region are not necessarily known, where transport of the tags to and from the communication region is not necessarily restricted and where contentions among communications to and from tags need to be resolved in a time and energy efficient manner.
Tags can be attached to items such as parts, equipment, furniture, vehicles or to persons, to livestock or to any object having requirements to be tracked, located, or identified. Communications with the tags may be for the purposes of inventory, stock location in warehouses, determination of work-in-progress status, environment history, personnel location or for other purposes necessary for the efficient operation of a process. Examples of such processes are manufacturing, warehousing, inventory management, storage and transfer facilities and personal communication systems.
The communication region in which the communication occurs to identify, locate or track tagged items may be small or large, cellular or single celled or may have other characteristics. For example, in inventory or tracking applications, items may be in widely separated locations in a warehouse or may be grouped closely together in a small storage container. Also, tags may be attached to compound items having components where each component is itself a tagged item. Examples of such compound items are manifests, bills of lading, manufacturing travelers, transport trailers, containers or similar elements.
Communication systems have been developed for locating, tracking or identifying tags. For example, simple written lists or automated machines such as bar code readers or similar devices have been implemented for inventory tasks. The procedure for communicating with each of the tags can be simple if there are a relatively few tags or if the tags are readily accessible. Simple procedures usually have either a reader or other device brought to the tags or have the tags brought to the device. Such procedures require knowledge of the location of each item. If the location of an item is not known, the item must first be located before the procedure can occur. A significant mount of time may be required for those items positioned in relatively inaccessible areas such as a box interior or a remote shelf location.
Simple procedures may be adequate for small numbers of items or for items whose location is known or where the tag is easily accessible, however, for large numbers of items or if repositioning, unpacking or disassembly of items is required (for example in order to gain access to compound items), the simple procedures become complex and time consuming. For large numbers of items, the methods that may work for small numbers of items are inadequate.
One application of an identification system for large numbers of tagged items is in connection with a manufacturing facility. For example, an airplane manufacturing facility will have a large inventory of airplane parts (thousands of items) stored in a warehouse. New parts will be regularly received into the warehouse while other parts will be regularly withdrawn from the warehouse for use in manufacturing the airplanes. From time to time, items within the warehouse will be moved from one location to another. In the identification system, the warehouse is the communication region and each item has an attached tag and the warehouse has one or more interrogators in one or more cells for communicating with all the tags in the warehouse. This is an example of a wide area tracking system.
Another use of an identification system is in connection with a parcel delivery service where many packages are transported from a sender to a receiver through trucks which are at different locations within cities and within other regions of the country or world. Each truck typically has many packages (hundreds of items) regularly received into the track while other packages are regularly withdrawn from the truck for delivery. From time to time, items within the truck are moved from one location to another. In the identification system, each truck is a communication region and each item has an attached tag and the truck has one or more interrogators in one or more cells for communicating with all the tags in the truck. The entire fleet of trucks forms an expanded communication region where each truck is a separate region and the regions are not necessarily contiguous.
Still another use of an identification system is for work-in-progress tracking. For example, an aircraft engine repair facility requires tracking the engine through the various stages of repair or overhaul, a process that may take a considerable length of time and require frequent moving and temporary storage of the engine. To manage this process requires an identification system that can locate, identify and track items in an unstructured environment.
Another use of an identification system in an unstructured environment is for sensor monitoring. For example, temperature gauges can be attached to items distributed in one or more regions of a cold storage facility where it is desirable to periodically monitor the temperature at each gauge and to note the temperature or whether or not an alarm temperature or other parameter has been exceeded. An effective communication system is required in order to identify and interrogate the numerous items without the necessity of having to locate or access each of the items individually, thus allowing the freedom to relocate items from time to time without regard to the ability to interrogate the items at a future time.
Thus a need exists for an accurate and efficient system that locates, identifies, tracks or otherwise communicates with large numbers of items. The system must operate in a time and energy efficient manner without the need to unpack or disassemble storage containers, without the need to be physically close to the items and without the need for a rigorous manual bookkeeping system to keep track of the items and their status, particularly if the items are moveable within a region.
A number of communication systems exist for identification, location or tracking of items and these systems are based upon many different technologies. Some of the communications may be in a broadcast mode (one to many) where an interrogator broadcasts to many tags, others may be in a one-to-one mode where communication is between one interrogator and one tag. Examples of one-to-one systems are optical bar code readers, optical character readers and magnetic stripe readers, all commonly used in identification systems. The communication range of these systems is typically less than a meter, limiting their application to use where the tags are in close proximity to a reader. Because such systems require close proximity for interrogation, they are of little value when the location of the item is unknown or when it is desired to communicate over a larger communication region such as a warehouse, a truck or other large region.
Radio frequency (RF) identification systems have been used for identification and tracking where an increased range, relative to the short range of proximity detectors, is required. One type of RF system uses magnetically coupled tags affixed to the items to be identified. In such magnetic coupling systems, tags are energized by movement of the tag through a magnetic field generated by an interrogator and the energized tags magnetically couple energy back to the interrogator. These systems find application in inventory control where items pass through portals. For instance, shrink-wrap packages such as computer software or tagged clothing in retail establishments effectively use magnetic coupling systems.
Magnetically-coupled tags are inherently restricted to close-range communications not extending beyond several meters because such tags use small loop antennas that operate at VLF frequencies that have a low coupling efficiency. While larger antennas are theoretically possible, in actual practice small loop antennas are required to avoid excessive tag size. This antenna size limitation precludes the use of such systems in other than small areas with small numbers of tags within the communication region. These systems are also not effective when the location of the tag is not known since, in these systems, the tag must be moved through the interrogation field of the reader thus necessitating prior knowledge of the tag location.
Another type of radio frequency (RF) system uses passive reflecting tags affixed to the items to be identified. When the items having tags are positioned within the range of the radiation radiated from an RF source, the tags are energized by the incident RF radiation at the tags. The tags modify the incident RF radiation and reflect a portion thereof back to a receiver at the RF source thereby producing an identification signal.
The passive RF reflector systems are energy efficient as the tags do not consume power, but instead simply reflect back incident radiation. However, there exist several inherent limitations in passive systems. In passive systems, the signal-to-noise ratio of the reflected identification signal is dependent upon the power level of the incident RF radiation at the tag, the geometry of the reflector and the efficiency of the modification and reflection operations. It is common for the reflected identification signal to be substantially weaker (for example, 100 db weaker) than the incident signal, and therefore, strong incident signals are required for the passive reflectors to work even over limited ranges of small regions.
To increase the power of incident radiation and thereby increase the range of a passive reflector system, passive reflector systems have employed focused radiation rather than omni-directional radiation since the incident power of focused radiation tends to be greater than the incident power of omni-directional radiation. Focused radiation, however, is not practical for a location system because it requires prior knowledge of the location and direction of the tag with respect to the transmitting source. Although reflective systems are used as verification or security systems, reflective systems have not proved practical for identification systems for items of unknown location or in an unstructured environment.
RF systems employing active communication between interrogators and tags are the most practical method to solve the identification task presented by a large number of tags in an unstructured environment. These active systems typically utilize broadcast techniques allowing a number of tags in an area to be located and identified by RF communication. Presently known active systems, however, although commonly used with small numbers of tags per reader, (typically less than ten), do not possess the orderly and efficient methods necessary to resolve the communication conflicts that arise in applications where large numbers of tags, typically hundreds or thousands, are present. Existing systems are not adequate where large numbers of tags are present and where the tags, in battery-powered operation for example, have a finite and small amount of energy available. Specific examples of proposed communication systems useful in limited environments are known.
Examples are communication systems wherein each tag responds to interrogation during a unique time slot, fixed by a tag address code, at a particular response time after interrogation without any provision for collision resolution. Such a system is limited to interrogating a small number of tags at one time or to only a fixed number of tags and is inadequate for large numbers of tags or for a varying population of tags because these systems have no provision for collision resolution.
Another example is a communication system for interrogating transient tags brought into the field of an interrogator where the interrogator sends a synchronization signal to responsive tags and identifies the responding tags with no acknowledgement to a tag to communicate to the tag that a successful transmission was received by the interrogator. In that system, the interrogator continuously broadcasts interrogation requests and listens for and records, when able, tags which respond. The collisions which inevitably result from two or more tags responding simultaneously to the interrogator are attempted to be overcome by having the tags indefinitely repeat their transmissions at randomly chosen times. Such a system tends to create an unacceptable collision problem in the case of many tags or stationary tags and hence is limited to identification of only a few tags and then only if the few tags are transient at the interrogator station.
As another example, a communication system uses two frequencies, one for interrogators to send and the other for tags to respond using various communication sequences. If more than one tag responds, the tag signals collide and the interrogator will detect errors and copy those errors back to the tags. The tags transmit again and frequently again collide repeating the error transmissions. The tags then go silent and respond again after a random time delay. Such a system is deficient in organizing the energy resource. The system is limited to only a few tags since if expanded to a large number of tags, the system presents an unacceptable level of energy consumption due to the disorganized method of resolving collisions. The response acknowledge cycle of each single tag, along with the associated time overhead in error determination, requires constant transmission of signals, consuming an excessive amount of power.
Still another communication system employs multiple frequency responses to interrogation where a transmitter transmits messages to a group of pagers. The transmitter transmits tag addresses in a time-multiplexed group on a single frequency to normally sleeping tags. All tags wake up and listen to the address to determine if their unique address is contained in the group and if so, at what relative position in the sequence. If a tag determines that its address is not in the group of addresses sent, the tag returns to sleep. If the tag determines that its address is in the group, it remains awake to receive a message sent by the central transmitter. Having received its message, the tag sends back to the central transmitter a response signal on a frequency specified by the relative position of the tag address in the group address transmission. Such a system limits the number of tags that can respond at any one time to the number of frequencies available for responses. Only a small number of frequencies, perhaps as few as twenty, are practical due to design tradeoffs between the number of frequencies needed and the frequency precision required of the transmitter and receiver design. The antenna design also becomes more expensive and complex due to the wide frequency bandwidth within which such systems must be responsive.
The communication systems described by way of example, and other proposed systems, are unable to satisfy the need to identify one or more of a plurality of tags within a given area, to resolve collisions in the responses of interrogated tags and to accomplish these tasks in a time and energy efficient manner for a large number of tags.
In view of the above background, there is a need for highly efficient communication systems capable of operation in an orderly and time and energy efficient manner with large numbers of tags to communicate with all tags for purpose of inventory or for other purposes.
For an effective communication system for communicating with items in a communication region to locate, track, or identify the items or to communicate with the items for other reasons, many factors must be considered including the following.
The size of the communication region determined in part by the communication range of the signals from interrogator to tags and from tags to interrogator.
The rate at which tags are introduced into and removed from the communication region.
The number of tags which are within the communication region at any one time where a large number may be hundreds or thousands or more and a small number may be none or a few.
The nature and number of communication channels between the tags and the interrogators.
The bandwidth of the communication channels between the tags and the interrogators.
The reliability of the communication channels.
The efficiency of time with which the interrogation process can be completed and the speed of communications.
The type of communication protocol that is employed.
The cost of the system and particularly the cost of each tag.
Power requirements including battery life and size for portable operation.
Additional desirable features of an identification system are the ability to increase the range of the system over a larger communication region by forming adjacent communication cells in a cellular system where each cell includes an interrogator that communicates with tags over a part of the larger communication region so that a plurality of such interrogators together effectively communicate over the entire communication region. Such a system, having coordinated communications among the cells, defines a wide area identification or asset tracking system.
In summary, efficient communication systems are needed that take inventory of, or for other purposes communicate with, tags within a communication region. Since the number of tags may be hundreds or thousands, the communication protocol is significant and must consider cost, reliability, accuracy, energy efficiency and the other factors identified above. Also, since tags are transportable when attached to transportable items, the tags are typically battery operated and hence the need to conserve power in order to extend battery life is of major consideration.